linux-kernel - Re: [PATCH v9 08/12] mm: zero reserved and unavailable struct pages

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Date:   Wed, 4 Oct 2017 08:40:11 -0400
From:   Pasha Tatashin <pasha.tatashin@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, kasan-dev@...glegroups.com, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com, davem@...emloft.net,
        willy@...radead.org, ard.biesheuvel@...aro.org,
        mark.rutland@....com, will.deacon@....com, catalin.marinas@....com,
        sam@...nborg.org, mgorman@...hsingularity.net,
        steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        bob.picco@...cle.com
Subject: Re: [PATCH v9 08/12] mm: zero reserved and unavailable struct pages

>>> Could you be more specific where is such a memory reserved?
>>>
>>
>> I know of one example: trim_low_memory_range() unconditionally reserves from
>> pfn 0, but e820__memblock_setup() might provide the exiting memory from pfn
>> 1 (i.e. KVM).
> 
> Then just initialize struct pages for that mapping rigth there where a
> special API is used.
> 
>> But, there could be more based on this comment from linux/page-flags.h:
>>
>>   19  * PG_reserved is set for special pages, which can never be swapped out.
>> Some
>>   20  * of them might not even exist (eg empty_bad_page)...
> 
> I have no idea wht empty_bad_page is but a quick grep shows that this is
> never used. I might be wrong here but if somebody is reserving a memory
> in a special way then we should handle the initialization right there.
> E.g. create an API for special memblock reservations.
> 

Hi Michal,

The reservations happen before struct pages are allocated and mapped. 
So, it is not always possible to do it at call sites.

Previously, I have solved this problem like this:

https://patchwork.kernel.org/patch/9886163

But, I was not too happy with that approach, so I replaced it with the 
current approach as it is more generic, and solves similar issues if 
they happen in other places. Also, the comment in page-flags got me 
scared that there are probably other places perhaps on other 
architectures that can have the similar issue.

In addition, I did not like my solution, I was simply shrinking the low 
reservation from:
[0 - reserve_low) to [min_pfn - reserve_low), but if min_pfn > 
reserve_low can we skip low reservation entirely? I was not sure.

The current approach notifies us if there are such pages, and we can 
fix/remove them in the future without crashing kernel in the meantime.

Pasha